332 THE PRODUCTION OF SULPHUR DIOXIDE

the working-doors, which determines the depth of the layer of
pyrites, varies from I ft to 2 ft. The height from the upper
level of the ore or to the abutment of the arch is usually
about equal to that of the working-door, say 9^ to 12 in., and
from there up to the crown of the arch another 8 or 9 in.
The arch itself is either sprung from side to side, as is the
custom on the Tyne (whereby the walls are made to bear the
weight more equally and the working through the doors is
facilitated), or, as is usual in Lancashire, from front to back
(which is more advisable in the case of two rows of burners
being built back to back, in which case the arch is sprung
over both burners together, with a supporting wall in the
centre). In any case it is advisable to build the burners
back to back, even with arches sprung from side to side, wher-
ever it is locally possible ; thus one back wall is saved, the
heat is kept up better, and a common gas-flue can be employed.

The gas-flue of the English burners is always at the top,
each burner-arch having a hole of 4 to 5 in. square leading
into it. These holes are not always provided with dampers ;
but by gradually increasing the size of the holes as the distance
from the main shaft becomes greater, regularity of draught is
produced. The flue itself can be made of bricks, and covered
with fire-tiles. Most modern works prefer forming it by a
second arch, about 6 or 12 in. above the burner-roof, reaching
right across the whole burner, and supported by the front plate
being made high enough.

The principal feature of the English pyrites-burners, which
is found in all Continental works as well, except in some burners
for metallurgical purposes (Mansfeld or Freiberg kilns), is the

FIG. 32.

employment of grate-bars of square section, movable in bearings,
and leaving larger or smaller spaces between them, according
to their position. Fig. 32 represents such a grate-bar, showing
also the parts which are forged or machined with a circular
section, so that they can easily turn in the respective bearers.
Bars 2 in. square are usually made of wrought-iron; the